drm/amd/display: fix a potential issue in DSC logic

[why]
In compute dsc bandwidth range there is an uninitialized variable

[how]
Initialize the variable to the correct value.

Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Nikola Cornij <Nikola.Cornij@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>